Which of the following can affect the effectiveness of air purifying respirators?

The effectiveness of air purifying respirators is significantly influenced by the concentration of contaminants in the environment. These respirators function by filtering out harmful particulates and gases present in the air. If the concentration of contaminants is too high, it can overwhelm the filtration system, making it unable to adequately clean the air that the wearer breathes.

In scenarios where the concentration of hazardous materials exceeds the respirator's rated capacity, the user may still be exposed to those harmful substances, leading to potential health risks. Therefore, it's critical to assess the contaminant levels in the environment to determine if the respirator is appropriate for use and to ensure that it will provide the necessary protection.

While factors like air pressure, weather conditions, and time of day can influence other environmental aspects, they do not directly impact the fundamental operation of air purifying respirators in the way that contaminant concentration does.